This book develops the general principles of linguistic change that form the foundations of historical linguistics, dialectology and sociolinguistics. It is concerned with the factors that govern the internal development of linguistic structures: the mechanisms of change, the constraints on change, and the ways in which change is embedded in the larger linguistic system. While it is generally believed that linguistic change is a product of earlier times, and that local dialects are disappearing, this work shows that rapid change is in progress in the cities of America and England, so that urban dialects are becoming more and more differentiated. Instrumental studies of these changes develop a new view of phonological space that allows the resolution of long-standing paradoxes of historical linguistics. The book then develops the general principles governing mergers and splits, which alter linguistic structure.